Before I get started with a few hasty remarks on today’s events, let me remind you that Lonnie Thompson’s Frontiers in Geophysics lecture will be webcast live on Wednesday at 1815 Pacific time. A link to the webcast can be found here. The lecture is entitled “Abrupt Climate Change and Our Future”. At the same page you’ll find links to Arvidson’s Whipple lecture on Mars exploration, which will be webcast at 14:20. Enjoy! Wish you were here.

Now, let me say at once how inspiring it is to see so much first-rate innovative science arrayed here. There are a lot of geophysicists in the world, and most of them are very, very good. It is especially encouraging to see so much new, young talent in all areas. I spend all too much of my time on RealClimate writing about bad science, it is great to come here and get a reality check.

Welcome, dear readers. For all of you who have eagerly been awaiting Part II of Les Chevaliers, thank you for your patience; with all the other interesting stuff coming up for discussion at RealClimate, the plans to post Part II ran up against the American Geophysical Union Fall Meeting, when 15,000 of our prime audience are holed up in San Francisco trying to decide which of a half dozen simultaneous sessions at any time best command their attention. Be of good cheer — Part II will be coming along in about another week. Meanwhile, Yours Truly offers a few off the cuff dispatches giving a personal and unedited view of a few things going on at AGU that may be of interest to the RealClimate readership.Myself, I have been spending a lot of time looking at some of the exciting new data coming in from planetary missions, but I’ll spare you that, and talk about things related to global change. I do not pretend that these are necessarily the most important things going on at the meeting, but they are a few things that I happen to have attended, and which caught my attention.

Recently, I received multiple requests to discuss a paper, due to appear in Journal of Geophysical Research (JGR-atmosphere), that has been presented in the media just before the Bali conference and the Nobel Peace prize ceremony here in Oslo, Norway. The paper concludes that the warming measured over land is most likely exaggerated due to non-climatic effects, and it presents a regression analysis suggesting that the real (climatic) global mean temperature trend should be ~50% lower over land.

Many people hold the mistaken belief that reconstructions of past climate are the sole evidence for current and future climate change. They are not. However, they are very interesting and useful for all sorts of reasons: for modellers to test out theories of climate change, for geographers, archaeologists and historians to examine the impact of climate on past civilizations and ecosystems, and for everyone to get a sense of what climate is capable of doing, how fast it does it and why.

As a small part of that enterprise, the climate of the medieval period has received a very high (and sometimes disproportionate) profile in the public discourse – due in no small part to the mistaken notion that it is an important factor for the attribution of current climate change. Its existence as a period of generally warmer temperatures (at least in the Northern hemisphere) than the centuries that followed is generally accepted. But the timing, magnitude and spatial extent are much more uncertain. All previous multiproxy reconstructions indicate a Northern Hemisphere mean temperature less than current levels, though possibly on a par with the mid- 20th century. But there are only a few tenths of a degree in it, and so the description that it is likely to have been warmer now (rather than virtually certain) is used to express the level of uncertainty.

A confounding factor in discussions of this period is the unfortunate tendency of some authors to label any warm peak prior to the 15th Century as the ‘Medieval Warm Period’ in their record. This leads to vastly different periods being similarly labelled, often giving a misleading impression of coherence. For instance, in a recent paper it was defined as 1200-1425 CE well outside the ‘standard’ definition of 800-1200 CE espoused by Lamb.

Since a new ‘reconstruction’ of the last 2000 years from Craig Loehle is currently doing the rounds, we thought it might be timely to set out what the actual issues are in making such reconstructions (as opposed to the ones that are more often discussed), and how progress is being made despite the pitfalls.More »

This post announces my (William Connolley’s) departure from RealClimate, and indeed from the professional climate field in general, in favour of the wide world of Cambridge software engineering. I’ve enjoyed my time with (Real)Climate, but now its time to move on.